335 research outputs found
Coagulation and fragmentation dynamics of inertial particles
Inertial particles suspended in many natural and industrial flows undergo
coagulation upon collisions and fragmentation if their size becomes too large
or if they experience large shear. Here we study this coagulation-fragmentation
process in time-periodic incompressible flows. We find that this process
approaches an asymptotic, dynamical steady state where the average number of
particles of each size is roughly constant. We compare the steady-state size
distributions corresponding to two fragmentation mechanisms and for different
flows and find that the steady state is mostly independent of the coagulation
process. While collision rates determine the transient behavior, fragmentation
determines the steady state. For example, for fragmentation due to shear, flows
that have very different local particle concentrations can result in similar
particle size distributions if the temporal or spatial variation of shear
forces is similar.Comment: 8 pages, 7 figure
A vertically resolved model for phytoplankton aggregation
This work presents models of the vertical distribution and flux of phytoplankton aggregates, including changes with time in the distribution of aggregate sizes and sinking speeds. The distribution of sizes is described by two parameters, the mass and number of aggregates, which greatly reduces the computational cost of the models. Simple experiments demonstrate the effects of aggregation on the timing and depth distribution of primary production and export. A more detailed ecological model is applied to sites in the Arabian Sea; it demonstrates that aggregation can be important for deep sedimentation even when its effect on surface concentrations is small, and it presents the difference in timing between settlement of aggregates and fecal pellets
Effect of type and concentration of ballasting particles on sinking rate of marine snow produced by the Appendicularian Oikopleura dioica
Ballast material (organic, opal, calcite, lithogenic) is suggested to affect sinking speed of aggregates in the ocean. Here, we tested this hypothesis by incubating appendicularians in suspensions of different algae or Saharan dust, and observing the sinking speed of the marine snow formed by their discarded houses. We show that calcite increases the sinking speeds of aggregates by ~100% and lithogenic material by ~150% while opal only has a minor effect. Furthermore the effect of ballast particle concentration was causing a 33 m d(-1) increase in sinking speed for a 5Ă10(5) ”m(3) ml(-1) increase in particle concentration, near independent on ballast type. We finally compare our observations to the literature and stress the need to generate aggregates similar to those in nature in order to get realistic estimates of the impact of ballast particles on sinking speeds
How Cooper pairs vanish approaching the Mott insulator in Bi2Sr2CaCu2O8+d
The antiferromagnetic ground state of copper oxide Mott insulators is
achieved by localizing an electron at each copper atom in real space (r-space).
Removing a small fraction of these electrons (hole doping) transforms this
system into a superconducting fluid of delocalized Cooper pairs in momentum
space (k-space). During this transformation, two distinctive classes of
electronic excitations appear. At high energies, the enigmatic 'pseudogap'
excitations are found, whereas, at lower energies, Bogoliubov quasi-particles
-- the excitations resulting from the breaking of Cooper pairs -- should exist.
To explore this transformation, and to identify the two excitation types, we
have imaged the electronic structure of Bi2Sr2CaCu2O8+d in r-space and k-space
simultaneously. We find that although the low energy excitations are indeed
Bogoliubov quasi-particles, they occupy only a restricted region of k-space
that shrinks rapidly with diminishing hole density. Concomitantly, spectral
weight is transferred to higher energy r-space states that lack the
characteristics of excitations from delocalized Cooper pairs. Instead, these
states break translational and rotational symmetries locally at the atomic
scale in an energy independent fashion. We demonstrate that these unusual
r-space excitations are, in fact, the pseudogap states. Thus, as the Mott
insulating state is approached by decreasing the hole density, the delocalized
Cooper pairs vanish from k-space, to be replaced by locally translational- and
rotational-symmetry-breaking pseudogap states in r-space.Comment: This is author's version. See the Nature website for the published
versio
Wild Felids as Hosts for Human Plague, Western United States
Plague seroprevalence was estimated in populations of pumas and bobcats in the western United States. High levels of exposure in plague-endemic regions indicate the need to consider the ecology and pathobiology of plague in nondomestic felid hosts to better understand the role of these species in disease persistence and transmission
Influence of plankton community structure on the sinking velocity of marine aggregates
About 50âGt of carbon is fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density, where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25âm deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ă 80â400â”m) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms, which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity (Pestimated) decreased as the picoautotroph (0.2â2â”m) fraction of the phytoplankton biomass increased. Thus, picoautotroph-dominated communities may be indicative for food webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500âcells/mL accelerate sinking by about 35â40%, which we estimate (by one-dimensional modeling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging; both of which are controlled by plankton community structure
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